An optical device having a Paschen-Runge mounting arrangement comprises a concave diffraction grating which receives through an inlet slit the polychromatic light emitted by the excitation of the sample to be analyzed. The inlet slit is placed onto the Rowland circle which is tangential to the concave spherical surface on which the grating is formed and which has a diameter equal to the radius of curvature of that surface. The monochromatic bundles, supplied by the grating, converge in points situated on the Rowland circle. The chosen wavelengths are selected by the outlet slits placed on the Rowland circle.
In the Paschen-Runge mounting arrangements, the slits, placed on the Rowland circle, are generally mounted individually onto a rigid slit-carrier which can be regulated, relative to the frame. This fitting is not well adapted for regulating the outlet slits.